I want to remove the bricks from the part of the stack that is in an upstairs room so I can temporarily feed some ducting into it? I will be removing an area of about a Sq Foot

I assume you never use this for any real fire or heat, it is merely a old unused chimmey

Is this an active chimney?If so the answer is no. Will let fumes from use below into upper area.

If inactive, would recommend blocking at lower inlet so as it can not be used in future.

Chuck